1. Technical Field
The present invention relates generally to a composite membrane and a method of making the composite membrane. In particular, the present invention relates to a porous membrane that is treated to provide oleophobic properties to the membrane and to a method of treating the membrane.
2. Description of the Prior Art
It is known that technical fabrics must be suitable for use in demanding applications. Examples of such demanding applications include filter elements, outerwear garments and apparel, footwear, tents, sleeping bags, protective garments, clean room garments, surgical drapes, surgical gowns, other types of barrier wear and allergen barrier products. The technical fabrics often include a film or membrane to protect the fabric user from an external condition or environment and/or protect the external environment from contamination by the user. The film or membrane may be made from any suitable material or structure and in any suitable manner.
A known material for the membrane that has proven particularly suitable for such demanding applications is made of an expanded polytetrafluoroethylene (xe2x80x9cePTFExe2x80x9d) material. The ePTFE membrane is typically microporous and laminated to at least one other material, such as a textile base or shell fabric. The resulting membrane and fabric laminate can be used to manufacture any number of finished products to meet the demands of the particular application.
It is known that an ePTFE membrane is air permeable and moisture vapor transmissive, yet resistant to wind and liquid penetration at moderate pressures. However, the ePTFE membrane tends to absorb oils and certain contaminating agents, such as body oils contained in perspiration, fatty substances or detergent-like contaminants. When the ePTFE membrane becomes contaminated by absorbing the oils or other contaminating agents, the membrane may no longer effectively resist liquid penetration.
One known approach to rendering an ePTFE membrane resistant to contamination by absorbing oils or contaminating agents includes applying a layer of polyurethane onto, or partially into, the ePTFE membrane, as disclosed in U.S. Pat. No. 4,194,041. A membrane with a polyurethane layer is generally contaminating agent resistant and has relatively high moisture vapor transmission rates. However, air may not freely permeate through the polyurethane layer. It is known that some degree of air permeability is desirable to increase user comfort. It is also known that the polyurethane layer must be wet in order to effectively transmit moisture vapor which can feel cold, wet and xe2x80x9cclammyxe2x80x9d to the user.
Another known approach to contamination resistance is to coat surfaces defining the pores in a porous membrane with a fluoroacrylate monomer and a polymerization initiator, as disclosed in U.S. Pat. No. 5,156,780. The initiator is activated to polymerize the monomer in situ to coat surfaces defining the pores in the membrane. This approach provides a membrane that is somewhat air permeable and resistant to absorbing oils and contaminating agents. However, this approach requires relatively expensive equipment and materials, such as an ultraviolet curing station and a nearly oxygen-free or inert atmosphere, to process and polymerize the monomer once it is applied to the membrane. Furthermore, this approach requires solvents, cross-linking reactants and/or initiators that may be environmentally unsound and difficult to obtain.
Yet another known approach is to coat a microporous membrane with an organic polymer having recurring pendant fluorinated organic side chains, as disclosed in U.S. Pat. No. 5,539,072. An aqueous dispersion carries the polymer and cannot enter the pores of the membrane. A relatively expensive fluorosurfactant is used in amounts that may be difficult to completely remove from the membrane in order to permit the polymer in the aqueous dispersion to enter the pores of the membrane.
Thus, a need exists to provide a membrane that is air permeable, moisture vapor transmissive, wind and liquid penetration resistant, durably resists absorbing oils and certain contaminating agents, is relatively inexpensive and easy to manufacture, made from readily available materials and does not require relatively expensive equipment, processes or materials. There is also a need to limit the agglomeration and xe2x80x9csettlingxe2x80x9d of dispersed particles or xe2x80x9csolidsxe2x80x9d in a coating that is to be applied to the membrane for a predetermined duration.
The present invention is directed to a method of treating a membrane. The method comprises the step of providing a membrane with surfaces that define a plurality of pores extending through the membrane. A dispersion of oleophobic fluoropolymer solids is provided. The dispersion is stabilized with a stabilizing agent. The dispersion is diluted with a wetting agent. Surfaces which define the pores in the membrane are wetted with the diluted and stabilized dispersion. The wetting and stabilizing agents are removed from the membrane. The oleophobic fluoropolymer solids of the dispersion are coalesced on surfaces that define pores in the membrane.
The step of providing a membrane comprises providing a microporous membrane, such as a membrane made from expanded polytetrafluoroethylene. The diluting step comprises diluting the stabilized dispersion in a wetting agent so the diluted and stabilized dispersion has a surface tension and contact angle relative to the membrane such that the diluted and stabilized dispersion is capable of wetting surfaces defining the pores in the membrane. The step of stabilizing the dispersion comprises providing the stabilizing agent in a weight amount in the range of 0.5 to 5 times the weight of the wetting agent. The step of stabilizing the dispersion includes providing water as the stabilizing agent.
The step of providing a dispersion of oleophobic fluoropolymer solids comprises providing a dispersion of acrylic based polymer with fluorocarbon side chains. The step of diluting the dispersion of oleophobic fluoropolymer solids with a wetting agent comprises providing the wetting agent in a weight amount in the range of 2 to 10 times the weight of the dispersion. The coalescing step comprises heating the oleophobic fluoropolymer solids to a temperature in the range of 200xc2x0 C. to 240xc2x0 C. for at least ten seconds to flow and coalesce the oleophobic fluoropolymer solids on surfaces defining the pores in the membrane without completely blocking the pores.
The present invention is also directed to a composite membrane. The composite membrane comprises a porous membrane having a plurality of interconnecting pores extending through the membrane and made from a material that tends to absorb oils and certain contaminating surfactants. A coating is disposed on surfaces of the nodes and fibrils defining the interconnecting passages in the membrane. The coating comprises oleophobic fluoropolymer solids coalesced on surfaces of the nodes and fibrils to provide oil and surfactant resistance to the resultant composite membrane without completely blocking pores in the membrane. The membrane is gas permeable, liquid penetration resistant and capable of moisture vapor transmission at a rate of at least 70,000 gr/m2day.
The composite membrane includes the membrane being made from expanded polytetrafluoroethylene. The composite membrane is gas permeable at a rate of at least 0.10 cubic feet per minute per square foot.